Vacuum workholder and valve means therefor



D. F. MUIR. JR

Jan. 18, 1966 VACUUM WORKHOLDER AND VALVE MEANS THEREFOR Filed July 15, 1964 2 Sheets-Sheet 1 DOUGLAS F. MUlR JR =3 4 a I T zww/Ammm A TTORNEYJ D. F. MUIR, JR 3,229,953

2 Sheets-Sheet 2 3 m3 8N R. 2 Eu 6% m m an I y E 1 T m fi w J r J 2N MNQMW W F. N; @m an H a e a; k M Q b E I u g m wwm iii in Mwfl gw awn M MM, p m3 5 m M Jan. 18, 1966 VACUUM WORKHOLDER AND VALVE MEANS THEREFOR Filed July 15, 1964 ATTORNEY United States Patent 3,229,953 VACUUM WORKHOLDER AND VALVE MEANS THEREFOR Douglas F. Muir, .lr., 1511 Crest Road, Silver Spring, Md. Filed July 15, 1964, Ser. No. 382,760 6 Claims. (Cl. 248-363) This invention relates generally to grippers, holders and friction-increasing devices and more specifically to such devices utilizing vacuum to provide the operative force therefor.

There are many devices in the prior art for gripping items to lift, move or hold the items for performing various operations thereon. These past art devices include clamps, grapples, vises, forks, tongs, electromagnetic holders, vacuum lifts or holders and the like.

The vacuum type of device has been found to be particularly adaptable for the above uses and is especially valuable for holding sheets of material in a flat, planar condition. For this reason, vacuum operated devices have been widely used in photocopying processes and for holding film material in cameras where a planar disposition of the film is particularly essential.

This invention relates to improvements in the abovedescribed vacuum-powered devices. These devices generally have a holding surface shaped to conform to or conform the item to be held, the surface being provided with a series of on'fices over the entire area thereof to apply vacuum to the item to be held. A chamber, connected to'a source of vacuum, is disposed behind the holding surface to provide a vacuum reservoir for the orifices.

For all of the above-listed uses where vacuum grips or supports have been used, a particular difiiculty has been found in adapting them to hold objects of varying sizes or irregular edge contours. The use of such holders for handling objects of varying size of shape inevitably results in open orifices on the vacuum holder or gripper, not covered by the article being held or lifted if the article is smaller than the maximum capacity of the holder or because of its shape covers less than the whole area of the holder. This diminishes the overall holding force of the device when operating at less than full capacity.

Various attempts have been made in the past to adapt vacuum holders to avoid this problem where articles of varying size or shape must be held. Some of these devices provide zoned areas of orifices, the zones being isolated from one another with valving between the zones to provide communication therebetween when required. The valving in these cases is operated either manually or automatically to expose more zones and therefore larger numbers of orifices as the size or the configuration of the piece being held increases or changes.

This invention is particularly concerned with a vacuum holder or gripper of the last-described type wherein the orifices or groups of orifices for applying vacuum to the object to be held or gripped are arranged in isolated groups or zones to preserve the vacuum efficiency of the board disconnecting a zone from the source of vacuum when that zone is not required to hold the item on the holder. The holder is further of the type wherein pressure differential actuated valving is incorporated be- 3,229,953 Patented Jan. 18, 1966 tween the zones or groups of orifices to provide automatic communication between that zone and the other zones or group of orifices and/or the source of vacuum when that zone or group of orifices is required to provide holding of an item. This invention is further particularly concerned with such an arrangement wherein the valving means between the zones or groups of orifices comprises normally closed, spring-biased valves opened by a differential pressure between adjacent zones to furnish communication between the zones when one of the zones is substantially covered by the workpiece. As an example of a prior art valve system, to which the improvement of this invention would be applicable, the vacuum hold-er consists of a photocopy board having a plurality of orifices through the work holding face thereof, the orifices being communicative with a chamber behind the board. The chamber is divided up into a series of concentrically disposed, generally annular, channels formed by walls disposed in the chamber around a centermost channel. Vacuum is applied to the centermost channel and spring-loaded valves are provided through the walls defining each succeeding channel to successively open communication therebetween in series as size of the piece to be held increases. The disadvantage encountered in this and similar systems stems-from the fact that, as each valve is opened by the source of vacuum, the pump drawing the vacuum is required to maintain that valve open against a spring-biasing load. This increases the load on the pump for each valve being held open leading either to a limitation in the number of valves that can be used in a given system or to an increased capacity pump, the excess capacity of the pump being needed simply to hold the valves open. The size of the vacuum holder or the number of channels which may be included therein is thereby limited or an unnecessarily large pump is required for a given size system.

Another problem encountered in the above-described system is valve chatter or hunting caused by minute pressure fluctuations in conditions near the balance point between spring force and pressure differential force in the system. This condition is extremely undesirable in photocopy apparatus since the vibration caused thereby can cause loss of sharpness in focus and like problems.

It is an object of this invention to overcome the disadvantages of the prior art valves in vacuum devices of the type described by providing a valve which utilizes differential pressure between the interior of the holder and the atmosphere to hold the valve open so that, once open, the valve bias does not impose a load on the vacuum source of the system.

It is another object of this invention to provide a valve for vacuum holder systems which is opened by differential pressure between the chambers therein but which shifts, during the opening cycle, to actuation by differential pressure between the chamber and the atmosphere to relief the source of vacuum from the load required to hold the valve open.

It is still another object of this invention to provide an improved valve for vacuum holder systems which can be easily incorporated in existing systems without requiring extensive reworking thereof.

These and other objects of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in the light of the accompanying drawings in which like reference numerals indicate like parts in the figures thereof and wherein:

FIGURE 1 is a perspective view of a vacuum holder, the face thereof being partly broken away, illustrating the disposition of valves operating in accordance with this invention;

FIGURE 2 is a fragmentary perspective view of a portion of a vacuum holder of FIGURE 1 partly exploded to show a valve constructed in accordance with the invention;

FIGURE 3 is a sectional elevation of a view of another valve in accordance with the invention;

FIGURE 4 is a sectional elevational view of still another valve in accordance with the invention;

FIGURE 5 is a reduced sectional view in elevation of a modified valve embodying the invention;

FIGURE 6 is a sectional view of a portion of the valve showing another embodiment in accordance with the invention; and

FIGURE 7 is a perspective view of another application of the device in accordance with the invention showing its incorporation in a vacuum cylinder. 7

Briefly, the invention provides a differential pressureoperated valve which is spring loaded in a normally closed condition to block intercommunication between a first and a second chamber within a chambered vacuum-holding device. The valve, when actuated by the difference in pressure between the chambers, provides intercommunication therebetween. This invention provides the improvement which consists of a valve follower piston attached to each valve, the follower having one side thereof vented to one of the chambers and the other side thereof vented to ambient or a third pressure in such a manner that the differential pressure between the ambient and the chamber takes over the load of the spring bias through the follower piston when the pressure in the chamber is decreased to such a degree with respect to the ambient that the pressure differential therebetween is sufficient to overcome the spring bias and hold the follower and the valve in an open condition.

Referring now to FIGURE 1 of the drawings for an example of a specific embodiment of the invention, a flat board type of vacuum holder incorporating valves in accordance with the invention is shown generally at 10 and is illustrated in a partially broken away condition and comprises a back member 12 and a face member 14 which, in combination with a series of edge members 15, are disposed and connected to form a chamber therebetween. Walls 18 and 20, disposed Within the chamber formed by the members, divide it up into chambers 22, 24, 26 respectively. The face member 14 is provided with a plurality of vacuum orifices 28 throughout the surface area of the board, which orifices furnish communication between the chambers and the atmosphere and thereby provide means for applying vacuum to the object to be held. The wall members 18 and 20 effectively divide the orifices 28 into zones corresponding to the chambers disposed therebehind to provide a means for controlling the number of orifices to which the vacuum is applied.

The center chamber 26 communicates with a vacuum pump 30 through a conduit 32 communicative therewith so that, when the pump is operating, air is continually withdrawn therefrom. Valves, shown generally at 34 and 36, are disposed through walls 20 and 18 respectively to provide for valve controlled intercommunication between the chamber 26, the intermediate chamber 24 and outer chamber 22. The valve 34 is spring biased in a closed condition when the pressure differential between chambers 26 and 24 is substantially equal but opens to provide intercommunication between these chambers when the pressure differential reaches a value sufficient to overcome the biasing force of the spring which holds the valve closed. The valve 36 operates, in a manner identical to that of the valve 34, to provide intercommunication between the chamber 24 and chamber 22 when that pressure differential reaches the proper value. The device, as thus far described, is broadly in accordance with prior art devices in which the number of orifices in communication with the vacuum pump is automatically increased, as the size of the object to be held occupies an increasing area of the holding face of the device.

Referring now to FIGURE 2 of the drawings, the valve 36 is shown in enlarged perspective view which is partly exploded to more clearly illustrate the components thereof. The valve comprises a valving member 38, the member being broken for clarity of illustration with the right portion thereof being shown in an exploded condition and the left portion thereof shown in a retracted condition. The valve is further made up of a valve stem bolt 40, a tubular valve stem 42, a valve body 44 having valve orifices 46 disposed therethrough, a valve stem bushing 48 and a valve biasing spring 50 coiled around the valve stem 42. The valve stem bolt 40 is connected to the valving member 38 by a nut 52 threaded over the end thereof so that, with washers suitably disposed as illustrated, the valve spring 50 is interposed between the end of the valve stem and the valve stem bushing to normally bias the valving member 38 against the valve body 44 to seal the valve orifices 46. A valve follower piston 54 is interposed between the head of the valve stem bolt 40 and the valve stem 42. The follower piston 54 is disposed in a valve follower cylinder 56 which seats on a circular boss 58 on the face of the valve body 44 and is sealed at the rear end thereof by a plate 60 shown with the right-hand portion in exploded configuration and the left-hand portion thereof in an assembled configuration. The assembly is attached to the valve body 44 by bolts 62 to sandwich the cylinder therebetween. The plate 60 is provided with a pair of bores 64 which are communicative with the interior of the cylinder 56 on the rearward side of the piston 54 and, in the assembled condition, with ambient pressure through the bores 66 in the back member 12. The cylinders 56 are further provided, proximate the ends thereof which abut the valve body 44, with ports 68 which provide communication between the interior of the cylinder forward of the piston 54 and the chamber 22.

The valve, when assembled, then, incorporates a single valving member 38 biased to close a pair of valve orifices- 46. The valving member is mounted on a pair of valve stems 40 which are connected to a pair of valve follower pistons 54 operating in a pair of valve follower cylinders 56. Each of the cylinders 56 is communicative with the atmosphere, on one side of the piston 54, through the bores 64 and 66 and with the chamber in which they are disposed, on the other side of the piston, through ports 68. In this manner one side of the valve follower piston is subjected to atmospheric pressure while the other side is subjected to the pressure in the chamber in which the valve followers are disposed.

In operation of the valve, as the number of vacuum orifices 28 communicative with the chamber 24 are increasingly covered by the articles of increased size, the pressure in the chamber 24 is lowered to a greater degree by the action of the pump 30 so that an increasing pressure differential is instituted between the chamber 22 and the chamber 24.

When this pressure differential reaches a value great enough to overcome the bias of the spring 50, the valving member 38 is drawn away from the valve body 44 and air from the chamber 22 is drawn into the chamber 24 through the valve orifices 46 thereby applying the action of the vacuum pump to the vacuum orifices which com municate with the chamber 22. With this valving mem-. ber movement, the valve follower pistons 54 also move. toward the valve body 44 within the valve follQWQI cylin;

ders 56. When an object, because of its size, overlaps the area defined by the wall 18, a portion of the vacuum orifices 28, communicative with the chamber 22, are covered thereby initiating a reduction of pressure in that chamber. The respective sizes and number of the valve orifices 46 and vacuum orifices 28 are such that, as long as all of the orifices 28 are uncovered, there is a minimal pressure differential between the chambers and the ambient pressure. When the size of the object being held increases to a sufiicient degree, the action of the pump pulling air through the orifices 46 continues to decrease the pressure in the chamber 22 increasing the pressure differential between the atmosphere and the chamber 22. When this diiferential reaches a value, which is large enough to overcome the biasing force of the springs 50, the ambient pressure acts, through the bores 66 and 64 against the valve follower pistons 54 to hold the valves open independently of the pressure difference between the chambers 22 and 24. This result is effected since the differential pressure between ambient and the chamber 22 is effectively applied across the piston to hold the valve open because the communication of the interior of the cylinder forward of the valve follower piston with the pressure in the chamber 22 through ports 68. In this manner, the force required to hold the valve open, initially placed upon the vacuum motor 30, is relieved therefrom and taken over by the differential between ambient and the pressure in the chamber 22. This action is identical in each of the valves in the system and, once the pressure dilferential in the individual chamber and the ambient reaches a degree sufiicient to overcome the spring bias, the pump is freed of the biasing load of the individual valve and is therefore capable of undertaking the load of opening the next valve when required.

As shown in the drawings, the valve body 44 comprises a portion of the wall 18. The joint between the valve body and the wall 18 is formed, on either side of the valve body, by a tongue and groove joint 70 which is formed to be pressure tight but allows the valve body 44 to be slidably removed from between the segments of the wall 18 for replacement or repair of the valves as necessary.

A benefit over prior art devices is also furnished in this invention through the dash pot effect provided by the orifices 64 and 68 in the cylinder 56. As was discussed above, the prior art spring-loaded valves tend to flutter when the pressure differential opening the valves is on the order of the spring force biasing them closed. The reduced orifices in the cylinder of the present invention efiectively meter airflow into and out of the cylinder and limit the velocity at which it can be taken in or expelled. This damps, through the retarding effect thereof on the piston, any tendency in the valve to oscillate or flutter at marginal pressure conditions.

It'should be understood that, as well as functioning from the center chamber out as illustrated, the device can function from the outer chamber inwards by reversing the valves and drawing vacuum from the outer chamber. The devices could also control chambers arranged in series, parallel or alternating disposition as opposed to the concentric disposition shown.

Referring now to FIGURE 3 of the drawings, a valve, generally indicated at 136, is shown in enlarged elevational section and comprises a modification over the valve 36 of FIGURES 1 and 2. The primary distinction between the valve of FIGURE 3 and the prior valves is in the bore 64 which, in the prior views, is shown disposed through the plate 60 communicating directly with bore 66 in a back member 12 is, in this case, commtunicative, through a flexible tubular member 166 and a side wall member 114, with the atmosphere.

In the detailed view of FIGURE 3, the full assembly of the valve components is shown and the parts therein, which are equivalent to those of FIGURES l and 2, are indicated by like numerals of the next higher order. In

FIGURE 3, an O ring 172 in disposed around the pistion 154 to provide slidable sealing contact between the piston and the inner periphery of the wall of the cylinder 156. The plate 160 is provided with a boss 174 similar to the boss 158 which is provided on the valve body 144. The boss 174 slidably fits within the rear end of the cylinder 156 as shown with an O ring 176 disposed therebetween to provide sealing thereof. Other 0 rings may be, of course, suitably disposed throughout the structure as required to provide proper sealing of the joints between the components of the valve.

The primary benefit realized by the provision of the modification of FIGURE 3 lies in the ease it provides in the mounting and dismounting of the components through the lower alignment tolerance requirement afforded by the flexible conduits 166.

A further modification of the valve system in accordance with the invention is shown in FIGURE 4 of the drawings wherein a single valve, shown generally at 236, operates a valving member 238. The spring load which biases the valve in a closed position, generally determines the area of the piston required to hold the valve in an open condition within the limits of the pressure differential which may be reasonably obtained in the system. In the configurations shown in the preceding figures, the area of the piston required was such that, with the desired geometry of the holder selected, the diameter of the cylinder for a single cylinder configuration would be too large to be accommodated within the depth of the holder. In the prior configurations, then, the required piston area was achieved by providing two piston-cylinder combinations to furnish the required area without exceeding the desired depth of the holder. In the embodiment of FIGURE 5, where the depth of the holder is sufiicient to accommodate a single piston-cylinder combination, a valve in accordance with that shown in FIGURE 3 may be provided. Parts corresponding to those of the previous figures are indicated in this figure by the same numerals only of the next higher series. The only further modification, other than the provision of the single cylinder instead of a dual cylinder device, is the addition of a clamp 262 to hold the plate 260 to the base of a flanged cylinder 256 and a series of screws 263 to fix the forward portion of the cylinder 256 to the boss 258. These alternate members replace the bolts 62 and 162 of the previous figures and maintain the various components of the system connected together.

In instances where the modification of presently existing vacuum holders is desired or where immediate accessibility to the valve system without necessitating the dismantling of the vacuum holder is desired or required, a valve such as that shown in FIGURE 5 may be provided. In this figure, the valves, shown generally at 334 and 336, are provided as an external unit encased in a box-like valve enclosure 378 which is divided by valve bodies 344 and 345 to form chambers in the enclosure. The chambers are communicative with chambers 322 and 324 through ports 384 and 386 respectively to provide communication therebetween. The operation of the valving members and followers as well as the operation of the remaining portion of the apparatus is, identical to that described in the above devices embodying the valve. Communication between the valve follower cylinder 356 and the holder chamber 324 provided through conduits and orifices such as at 368, while communication between the cylinder and the atmosphere is by way of conduits and orifices such as at 366.

This variation also provides a capability of skipping chambers to provide alternate live and dead chambers, if such a requirement is desired.

Referring now to FIGURE 6, a variation in the valve follower piston sealing arrangement is shown. The primary modification of this embodiment is the provision of a rolling diaphragm 472 in place of the O-ring-type sealing arrangement shown in the above-described configurations. The rolling diaphragm provides a positive sealing arrangement between the piston cylinder 456 yet eliminates the friction normally attendant with a more conventional type of sliding seal such as the O-ring seal shown in the previous figures. The functioning of the rolling-type diaphragm is well known in the art and the diaphragms are commercially available under such trademark names, as the Bellofram diaphragm.

As stated elsewhere, the elevational shape of holders embodying the valves of this invention may take any desired form. The board may be concave, convex or any other desired shape. As an example, in FIGURE 7 valves embodying features in accordance with the invention are shown incorporated in a cylinder 588 which is adapted to process or power continuous sheet material or belts 590 of varying widths. By applying vacuum to this cylinder, slippage of the sheet material with respect to the cylinder can be reduced thereby affording greater efliciency in the transmission thereof. By incorporating the valves of this invention in the cylinder, the area from which the vacuum is drawn can be automatically changed while realizing the aforedescribed benefits of the values.

The cylinder 588 is powered by a motor 592 turning shaft 594 which rotates the cylinder. The cylinder is made up of a tubular face member 514 having orifices 528 disposed therethrough and, with circular edge members 516, forms a cylindrical chamber divided into outer chambers 522 and inner chamber 526 by walls 520. Vacuum is applied to inner chamber 526 from a vacuum pump (not shown) through conduit 532. Valves 536 provide intercommunication between the outer chambers 522 and the inner chamber 526 in the same manner as described in the above embodiments, when the width of the sheet material or belts covers the orifices 528 communicating with the inner chamber 526 to a degree sufficient to open the valves. The number of chambers can, of course, be multiplied as desired -to handle any width of sheet material anticipated.

What has been described above is intended to be exemplary of embodiments and uses in accordance with the invention to enable those skilled in the art to practice the teaching thereof. It should therefore be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically set forth.

What is claimed as new and desired to be protected by Letters Patent of the United States is:

1. A device for controlling fluid communication between first and second pressure zones disposed in a third pressure environment comprising:

means including a valve interconnecting said zones;

biasing means urging said valve closed to normally block intercommunication between said zones;

means to apply a subpressure to between said first zone and means responsive to said subpressure to open the valve when said difference in pressure between said zones is sufiicient to overcome said biasing means and thereby provide intercommunication to the second zone to apply the subpressure in said first zone to said second zone;

means actuated by the differential pressure between second zone and the third pressure environment to hold said valve in the open position independently of the difference in pressure between said first and second zones.

2. A device for controlling fluid communication between first and second pressure zones surrounded by an ambient pressure environment comprising:

means including a valve interconnecting said zones;

a valving member, biasing means urging said valving member closed to normally block intercommunication between said zones;

means to apply a subpressure to said first zone and means responsive to said subpressure to open said member when the difference in pressure between said zones is sufficient to overcome said biasing means to thereby provide intercommunication to the second zone to apply the subpressure in said first zone to said second zone;

a valve follower associated with the said member to follow the motion thereof, means to apply the pressure differential between said second zone and the ambient pressure to said follower to immobilize said follower so that said valve is held in the open position independently of the difference in pressure between said first and second .zones when the lattermentioned differential is sufiicient to overcome said biasing means.

3. A device in accordance with claim 2 wherein said valve follower comprises a cylinder having closed ends thereto, a piston slid-able in the cylinder, and a valve stem connecting said valving member to said piston;

and wherein said means to apply said pressure differential comprises an orifice in said cylinder between said piston and said valving member disposed to provide communication between said cylinder and said second zone and a conduit disposed to provide communication between said cylinder on the opposite side of said piston from said orifice, and the ambient pressure.

4. A vacuum holder comprising:

a hollow body defining an enclosed space therein;

one face of said body comprising a vacuum holding surface having a perforate structure which provides communication between ambient pressure and said enclosed space;

a plurality of walls disposed in said body to divide said enclosed space into a plurality of chambers;

a source of vacuum communicative with at least one of said chambers;

means including a pressure operated valve to control intercommunication between each of said chambers and the succeeding chambers thereto, biasing means to normally close said valve and means operative to open said valve when the difference in pressure between a chamber and the succeeding chamber to which it provides intercommunication is suflicient to overcome said biasing means;

means to apply the pressure differential between the last-mentioned chamber and the ambient pressure to said valve to hold said valve in the open position independently of the difference in pressure between chambers when said latter-mentioned differential is sufficient to overcome said biasing means.

'5. A vacuum holder comprising:

a hollow body defining an enclosed space therein;

one face of said body comprising a vacuum holding surface having a perforate structure which provides communication between ambient pressure and said enclosed space;

a plurality of Walls disposed in said body to divide said enclosed space into a plurality of chambers;

a source of vacuum communicative with at least one of said chambers;

means including a pressure operated valve to control intercommunication between each of said chambers and the succeeding chambers thereto, biasing means to normally close said valve and means operative to open said valve when the difference in pressure between a chamber and the succeeding chamber to which it provides intercommunication is sufiicient to overcome said biasing means;

a valve follower associated with said valve to follow the motion thereof, means to apply the pressure differential between the last-mentioned chamber and the ambient pressure to said last follower to im- 9 1O mobilize said follower so that the valve is held in References Cited by the Examiner the open position independently of the difference UNITED STATES PATENTS in pressure between chambers when said latter-mentinned differential is sufficient to overcome said biasgPE 2 1gg ing means. I 1 eman 6. A vacuum board in accordance with claim 9 where- 5 2,782,574 2/1957 Copold 51235 in said valve follower comprises a cylinder having closed 2,814,233 11/1957 Anander 248363 ends thereto, a piston slidable in said cylinder, and a 2,910,265 10/1959 Anander 248353 valve stem connecting said piston to said valve; 2,914,289 11/1959 Schutt 248363 and wherein said means to apply said pressure dif- 10 2,950,730 8/1960 SYeTISSOH 137116-3 ferential comprises an orifice in said cylinder be- 2,998255 8/1961 LIPklns 279-3 tween said valve and said piston disposed to pro- FOREIGN PATENTS vide communication between said cylinder and said last-mentioned zone and a conduit disposed to provide communication between said cylinder, on the 15 other side of said piston from said orfiice, and the HAROLD D WHITE HEAD Pnmary Exammer' ambient pressure. ROBERT C. RIORDON, Examiner.

892,771 3/1962 Great Britain. 

4. A VACUUM HOLDER COMPRISING: A HOLLOW BODY DEFINING AN ENCLOSED SPACE THEREIN; ONE FACE OF SAID BODY COMPRISING A VACUUM HOLDING SURFACE HAVING A PERFORATE STRUCTURE WHICH PROVIDES COMMUNICATION BETWEEN AMBIENT PRESSURE AND SAID ENCLOSED SPACE; A PLURALITY OF WALLS DISPOSED IN SAID BODY TO DIVIDE SAID ENCLOSED SPACE INTO A PLURALITY OF CHAMBERS; A SOURCE OF VACUUM COMMUNICATIVE WITH AT LEAST ONE OF SAID CHAMBERS; MEANS INCLUDING A PRESSURE OPERATED VALVE TO CONTROL INTERCOMMUNICATION BETWEEN EACH OF SAID CHAMBERS AND THE SUCCEEDING CHAMBER THERETO, BIASING MEANS TO NORMALLY CLOSE SAID VALVE AND MEANS OPERATIVE TO OPEN SAID VALVE WHEN THE DIFFERENCE IN PRESSURE BETWEEN A CHAMBER AND THE SUCCEEDING CHAMBER TO WHICH IT PROVIDES INTERCOMMUNICATION IS SUFFICIENT TO OVERCOME SAID BIASING MEANS; MEANS TO APPLY THE PRESSURE DIFFERENT BETWEEN THE LAST-MENTIONED CHAMBER AND THE AMBIENT PRESSURE TO SAID VALVE TO HOLD SAID VALVE IN THE OPEN POSITION INDEPENDENTLY OF THE DIFFERENCE IN PRESSURE BETWEEN CHAMBERS WHEN SAID LATTER-MENTIONED DIFFENTIAL IS SUFFICIENT TO OVERCOME SAID BIASING MEANS. 